Conventional HID dimming circuits switch capacitive reactance to effect dimming in an HID lamp. An example of such a circuit is illustrated in FIG. 1. The dimming circuit 100 includes a ballast 102 having an input terminal 104, and output terminal 106, and a common terminal 108. A first capacitor 110 is connected between the ballast output terminal 106 and the common terminal 108. In order to turn a dimming effect on or off, a second parallel capacitor 112 is selectively connected between the ballast output terminal 106 and the common terminal 108 by a relay 114. The capacitors 110, 112 are preferably connected to the relay circuit 114 through capacitor connection terminals 116. The relay 114 is preferably a solid state relay, and typically includes control input terminals 118 to selectively activate the relay and thereby connect the second capacitor 112 to the circuit for full power operation of the lamp (lamp not shown).
When the second capacitor 112 is connected to the circuit, any charge stored in the first capacitor 110 dumps current into the second parallel capacitor 112 until the voltage across both capacitors is equal. This sudden rush of current can damage the circuit, and in particular the contacts 116 of the relay 114 that connect to the capacitor. This phenomenon is exacerbated by the low impedance typically used in HID dimming circuits. Therefore, there is a need to protect the circuit and the capacitor contacts 116 when switching the second capacitor 112 into the circuit.
Conventional lighting devices utilize a special semiconductor feature to switch the capacitive reactance when dimming lighting HID ballasts. This feature is known as zero-voltage switching or ZVS. During ZVS, the device waits for the alternating voltage at the switch contact points to cross zero voltage in order to minimize the onrush of current, prevent contact degradation, and to prolong the life of the switch. Another common practice is to place a snubber circuit in-line with the contacts of a switch to protect the contacts. This will also prolong the life of the switch contacts.
The switch is connected in parallel to the main circuit capacitor and will connect another dimming capacitor into the circuit for full power operation of the luminaire ballast. When the switch closes, any voltage in the main circuit capacitor will dump current into the newly established leg of the dimming capacitor branch. The inrush of current can be substantial if the voltage in the main capacitor is large. When a zero-crossing detection circuit is used in conjunction with a switch, the excessive inrush of current due to a charge stored in the first capacitor is avoided. However, in circuits that lack zero-crossing detection, another protection mechanism is needed.